Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Tread compositions typically contain one or more elastomers and one or more types of reinforcing materials such as particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of the desired properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of filler particles can be improved by increasing their interaction with the elastomer(s) and/or decreasing their interaction with each other. Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer, typically at a terminus thereof.
Terminal chemical modification often occurs by reaction of a living (i.e., anionically initiated) polymer with a functional terminating agent. Some of the numerous examples of this approach include U.S. Pat. Nos. 3,109,871, 4,647,625, 4,677,153, 5,109,907, 6,977,281, etc., as well as references cited therein and later publications citing these patents.
Terminal modification also can be provided by means of a functional initiator, in isolation or in combination with functional termination. Functional initiators typically are organolithium compounds that additionally include other functionality, typically functionality that includes a nitrogen atom, capable of interacting with one or more types of particulate filler materials.
Functional initiators generally have relatively poor solubility in hydrocarbon solvents of the type commonly used in anionic polymerizations. Further, many functional initiators also do not maintain propagation of living ends as well as more common alkyllithium initiators such as butyllithium. Both of these characteristics can negatively impact polymerization rate and efficiency.
At least some members of a new class of functional initiators described in WO 2009/086490 exhibit excellent solubility in hydrocarbon solvents, even aliphatic hydrocarbon solvents, and/or an ability to maintain propagation of living ends of polymer chains. Many of these compounds are most effective at only relatively moderate temperatures (e.g., below ˜80° C.), however, meaning that their use typically is limited to systems or processes where heat transfer can be best accounted for, e.g., large reactor vessels, moderate polymerization rates (i.e., longer polymerizations), etc., which tend to be characteristic of batch processes.